Deal covers a 65km section of track between Cardiff and Bristol Parkway
Balfour Beatty has won a contract to electrify a 65km stretch of railway as part of the Great Western Mainline upgrade.
The contract award follows a four-month early contractor involvement phase, working with Network Rail to agree scope, programme and cost.
The Great Western Mainline upgrade was criticised by MPs this year when costs on the £2.8bn scheme overran by £1.2bn in just a year.
Balfour will be responsible for the remainder of the electrification on section 6P, 8 and 9, between Cardiff and Bristol Parkway.
Main works are scheduled to start this month with completion expected towards the end of 2018. During peak construction, the project will employ over 300 workers.